This invention relates generally to rotary spray grainers and more particularly to the use of adjustable spraying means or nozzles in a spray grainer to optimize the spray's dispersion on the falling cascades of spray grained calcium hypochlorite.
The use of a rotary drum apparatus to apply coatings or encapsulations to particulate substrates by the spraying of slurries, melts, or solutions and suspensions onto moving beds of particles that fall in cascades within the apparatus is well known. The application of distinct and separate coatings to a particulate substrate is known as granulation and is widely used in the fertilizer industry, for example in the production of ammonium nitrate. The application of coatings or encapsulations on a particulate substrate can also be employed in the making of confections, such as sugars, or, as in the case of the instant invention, in the manufacture of calcium hypochlorite for use as a commercial bleaching and sanitizing agent, particularly in the disinfection of swimming pool waters.
Previous attempts to employ a rotary drum dryer in the process of manufacturing spray grained particles have employed either a deflector means that runs the entire length of the rotary drum and which overlies the spraying means or nozzles or, in the alternative, utilize no deflector means in the rotary drum. In both of these approaches the spraying means or nozzles have been permanently emplaced within the spraying zone of the rotary drum spray grainer so that the distance from the falling cascades of particles and the angle of spray of the nozzles or spraying means into the falling cascades have not been able to be varied. Similarly, the positioning of the nozzles or spraying means relative to the longitudinal axial length of the drum has been fixed so that the nozzles couldn't be adjusted laterally along or parallel to the longitudinal axis of the drum. These prior art designs prevent the rotary spray grainer from achieving its optimum operating efficiency.
In apparatus which does not employ a deflector plate and which is rotated at a sufficient speed to obtain a full cascade of falling particles across the entire cross-sectional area, the slurried material being sprayed on the particulate particles does not achieve optimum spray pattern dispersion to uniformly wet the particles. The slurry is formed from a solid or semi-solid that is reduced to a sprayable liquid state by melting or by dissolution or suspension in an appropriate solvent. Frequently in this type of apparatus, the slurry will penetrate the falling wall or cascade of particulate substrate or particles and build up on the interior walls of the drum, creating scaling. Alternately, in this type of apparatus the falling cascade of the particulate substrate will fall in a path that causes it to strike the spraying nozzles or spraying means, thereby causing the spraying means or nozzles to clog and build up an accumulation of the slurry being sprayed.
Another approach not utilizing a deflector plate or deflector means in a rotary spray grainer uses rods disposed substantially parallel to each other and to the longitudinal axis of the drum radially inwardly of the interior walls of the drum. These rods facilitate the falling of particles in cascades or curtains so that the agglomerating agent or slurry may be sprayed into the cascades. However, this design still suffers from the disadvantage of permitting the particles to impinge on the spraying nozzles and thereby cause clogging or to fall in cascades so close to the nozzles that localized overwetting occurs to some of the particles, while others are not sufficiently wet.
In rotary drum dryers or spray grainers not utilizing deflector plates or other deflector means, avoidance of the impingement of particles on the spraying means has been achieved by keeping the drum rotational speed sufficiently low. This permits the nozzles or spraying means to disperse the slurried material to achieve the optimum spray pattern dispersion, defined by the proper free spray distance which is the distance from the cascade to the end of the nozzle tip to uniformly wet the falling particles. This optimum spray pattern dispersion, however, is obtained only if the nozzles or spraying means are correctly positioned initially for the specific drum rotational speed. While this avoids the impingement problem, it adversely affects the quality of the spray grained product obtained and reduces the capacity of the dryer or spray grainer. Both of these results are clearly not desirable. Also, if the drum rotational speed is varied, the proper free spray distance for that speed may change because the cascade of falling particles may shift transversely with respect to the longitudinal axis of the drum due to the change in rotational speed of the drum and the particles within the drum. Since the nozzles or spraying means in the prior art devices are fixed in position, the optimum spray pattern dispersion then cannot be achieved.
In other rotary drum dryer apparatus using the spray graining principle, deflector means or deflector plates were provided that ran substantially the entire length of the drum or a portion of the length of the drum. These full or partial length deflector plates would protect the spraying means or nozzles from impingement by the falling cascade of particulate substrate and would avoid the concomitant clogging of the nozzles due to slurry buildup. The deflector plate also provides a dense cascade of falling particles at a fixed distance from the spray nozzles to permit a full slurry spray pattern to be obtained.
However, if the rotational speed of the drum is varied, the optimum free spray distance mentioned above will also vary. Generally, the faster the drum rotational speed, the further outward towards the outer wall will the cascades of sprayed particles fall. This, then requires the adjustment of the nozzles or spraying means in a direction transverse to the longitudinal axis of the rotary drum to control the strike of the spray on the cascade and the fineness of the spray mist at the point of contact of the spray with the cascade. Additionally, the angle of spray of the slurry may need to be adjusted so the spray strikes the falling cascade at the proper height to ensure thorough wetting. Also, the point of contact of the spray with the cascade along its axial length in a direction parallel to the longitudinal axis may require adjustment to ensure that optimum wetting during the spraying operation and optimum drying during the drying operation are achieved.
These problems are solved in the design of the present invention by employing variably displaceable nozzles or spraying means in a rotary drum spray grainer.